Kinetics and Mechanism of Metal Nanoparticle Growth via Optical Extinction Spectroscopy and Computational Modeling: The Curious Case of Colloidal Gold

Abstract

An overarching computational framework unifying several optical theories to describe the temporal evolution of gold nanoparticles (GNPs) during a seeded growth process is presented. To achieve this, we used the inexpensive and widely available optical extinction spectroscopy, to obtain quantitative kinetic data. In situ spectra collected over a wide set of experimental conditions were regressed using the physical model, calculating light extinction by ensembles of GNPs during the growth process. This model provides temporal information on the size, shape, and concentration of the particles, and any electromagnetic interactions between them. Consequently, we were able to describe the mechanism of GNP growth and divide the process into distinct genesis periods. We provide explanations for several longstanding mysteries, e.g., the phenomena responsible for the purple-greyish hue during the early stages of GNP growth, the complex interactions between nucleation, growth and aggregation events, and a clear distinction between agglomeration and electromagnetic interactions. The presented theoretical formalism has been developed in a generic fashion so that it can readily be adapted to other nanoparticulate formation scenarios such as the genesis of various metal nanoparticles. KeywordsGold nanoparticles, seeded growth, UV-Vis spectroscopy, computational modeling, kinetics and mechanism.reason, there is a great need for a general and easily adaptable theoretical framework that utilizes the practical application of OES in the study of nanomaterial formation.So far, several investigations have addressed the mechanistic aspects during the (seeded) growth of GNPs using various characterization techniques such as atomic force microscopy (AFM), 5,14,18 electrophoretic measurements, 19,20 redox potential/pH measurements, 18-20 dynamic light scattering (DLS), 14,19,21 ex situ TEM, 4,5,7,14,[19][20][21][22][23] in situ TEM, 4,5 , and X-ray scattering. 6,7,13,23,24 Many of these studies follow the process also using ex situ 5,[19][20][21][22] or in situ 14,23,24 UV-vis spectroscopy but the information is treated merely qualitatively.From the plethora of research, some of which was summarized above, we know that the processes of seeded growth is typically accompanied by nucleation of new particles. 5,25 This could either be in a homogeneous fashion, 25 or in the close vicinity of the already present seed surface 5 (so called true catalytic secondary nucleation 26 or, equivalently, near surface nucleation followed by particle mediated growth 5 ). Additional complications arise from the possibility of agglomeration/aggregation invoked in many studies to describe the transient enhanced extinction in the wavelength range 600-800 nm, namely, the temporary purple-greyish colour of the suspension. 7,[18][19][20]27 Biggs et al. 18 and Chow and Zukoski 19 explained this in the light of the reduced colloidal stability in the presence of Au(III) in solution. Later, Rodriguez-Gonzalez and co-workers noticed that a homogeneous Au(III)→Au(I...

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“…Adapted with permission. [ 61 ] Copyright 2019, American Chemical Society. (C) Online batch apparatus with a recirculating line connected to a SAXS cell.…”

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